Fuel injection method

ABSTRACT

A method is provided for fuel injection in a sequential combustion system comprising a first combustion chamber and downstream thereof a second combustion chamber, in between which at least one vortex generator is located, as well as a premixing chamber having a longitudinal axis downstream of the vortex generator, and a fuel lance having a vertical portion and a horizontal portion, being located within said premixing chamber. The fuel injected is an MBtu-fuel. In said premixing chamber the fuel and a gas contained in an oxidizing stream coming from the first combustion chamber are premixed to a combustible mixture. The fuel is injected in such a way that the residence time of the fuel in the premixing chamber is reduced in comparison with a radial injection of the fuel from the horizontal portion of the fuel lance.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No.PCT/EP2008/067581 filed Dec. 16, 2008, which claims priority to EuropeanPatent Application No. 07150153.0, filed Dec. 19, 2007, the entirecontents of all of which are incorporated by reference as if fully setforth.

FIELD OF INVENTION

The present invention concerns the field of combustion technology. Amethod is proposed, whereby MBtu fuels with highly reactive componentscan be safely and cleanly burned in a sequential reheat burner, as founde.g. in a gas turbine.

BACKGROUND

In standard gas turbines, the higher turbine inlet temperature requiredfor increased efficiency results in higher emission levels and increasedmaterial and life cycle costs. This problem is overcome with thesequential combustion cycle. The compressor delivers nearly double thepressure ratio of a conventional compressor. The compressed air isheated in a first combustion chamber (e.g. via an EV combustor). Afterthe addition of a first part, e.g. about 60% of the fuel, the combustiongas partially expands through the first turbine stage. The remainingfuel is added in a second combustion chamber (e.g. via an SEVcombustor), where the gas is again heated to the maximum turbine inlettemperature. Final expansion follows in the subsequent turbine stages.

In so-called SEV-burners, e.g. sequential environmentally friendlyv-shaped burners, generally of the type as for instance described inU.S. Pat. No. 5,626,017, regions are found, where self-ignition of thefuel occurs and no external ignition source for flame propagation isrequired. Spontaneous ignition delay is defined as the time intervalbetween the creation of a combustible mixture, achieved by injectingfuel into air at high temperatures, and the onset of a flame viaauto-ignition. A reheat combustion system, such as the SEV-combustionchamber, also called SEV-combustor, can be designed to use theself-ignition effect. Combustor inlet temperatures of around 1000degrees Celsius and higher are commonly selected.

For the injection of gaseous and liquid fuels into the mixing section ofsuch a premixing burner, typically fuel lances are used, which extendinto the mixing section of the burner and inject the fuel(s) into theoxidizing stream (22) of combustion air flowing around and past the fuellance. One of the challenges here is the correct distribution of thefuel and obtaining the correct ratio of fuel and oxidizing medium.

SEV-burners are currently designed for operation on natural gas and oil.The fuel is injected radially from a fuel lance into the oxidizingstream and interacts with the vortex pairs created by vortex generators,as for instance described in U.S. Pat. No. 5,626,017, thereby resultingin adequate mixing prior to combustion in the combustion chamberdownstream of the mixing section.

SUMMARY

The present disclosure deals with a method for fuel injection in asequential combustion system having: a first combustion chamber and,downstream thereof, a second combustion chamber. In between the firstand second combustion chambers is a premixing chamber having alongitudinal axis that includes at least one vortex generator. Locateddownstream of the vortex generator is a mixing section and a fuel lancehaving a vertical portion and a horizontal portion parallel to thelongitudinal axis provided within said mixing section. The fuel has acalorific value of 5-20 MJ/kg. In the mixing section, the fuel and theoxidizing stream coming from the first combustion chamber are premixedto a combustible mixture. The method includes injecting the fuel in sucha way that the residence time of the fuel in the mixing section isreduced in comparison with a radial injection of the fuel from thehorizontal portion of the fuel lance.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings preferred embodiments of the invention areshown in which:

FIG. 1 shows a schematic view of a sequential combustion cycle with twocombustion chambers;

FIG. 2 shows a section through the current design of a fuel lanceoperating on natural gas and oil used for injection into a mixingsection of a premixing chamber;

FIG. 3 schematically shows, in a section through an SEV-burner, therelative positions of the fuel lance, vortex generators and combustionchamber;

FIG. 4 shows, in a schematic view, a section through an SEV-burner, inwhich the injection method according to one of the preferred embodimentsof the present invention can be exercised, according to a preferredembodiment of the present invention. The MBtu fuel plenum locatedbetween the fuel lance and the combustion chamber as an additional fuelinjection device;

FIG. 5 schematically shows a section through line B-B of FIG. 3; FIG. 5a.) shows fuel jets being injected without any tangential component withrespect to the periphery of the fuel lance; FIG. 5 b.) shows fuel jetsbeing injected from the fuel lance tangentially with respect to theperiphery of the fuel lance tube in swirl direction; FIG. 5 c.) showsfuel jets being injected from the fuel lance tangentially with respectto the periphery of the fuel lance tube against swirl direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Introduction to theEmbodiments

Currently, burners for the second stage of sequential combustion aredesigned for operation on natural gas and oil. In light of the abovementioned problems, the fuel injection configuration should be alteredfor the use of MBtu-fuels in order to take into account their differentfuel properties, such as smaller ignition delay time, higher adiabaticflame temperatures, lower density, etc.

The objective goals underlying the present invention is therefore toprovide an improved stable and safe method for the injection of MBtufuel for the combustion in such second stage burners or premixingchambers as known for example from U.S. Pat. No. 5,626,017.

In other words, the present invention pursues the purpose by providing amethod for fuel injection in a sequential combustion system comprising afirst combustion chamber and downstream thereof a second combustionchamber, in between which at least one vortex generator (e.g. swirlgenerator as disclosed in U.S. Pat. No. 5,626,017) is located, as wellas downstream of the vortex generator a premixing chamber having alongitudinal axis, with a mixing section and a fuel lance having avertical portion and a horizontal portion, extending into said mixingsection. Said fuel lance can for instance be of the type disclosed in EP0 638 769 A2, or any other fuel lance type known in the state of theart. The fuel to be injected, preferably a MBtu-fuel, has a calorificvalue of 5,000-20,000 kJ/kg, preferably 7,000-17,000 kJ/kg, morepreferably 10,000-15,000 kJ/kg. In said premixing chamber, or in itsmixing section, respectively, the fuel and the oxidizing stream(combustion air) coming from the first combustion chamber are premixedto a combustible mixture. The fuel is injected in such a way that theresidence time of the fuel in the premixing chamber is reduced incomparison with a radial injection of the fuel from the horizontalportion of the fuel lance. Thereby, the creation of the combustiblemixture and its spontaneous ignition is postponed.

Experience from lean-premixed burner development indicates that the SEVburner has to be redesigned in order to cope with the radicallydifferent combustion properties of MBtu (MBtu fuel input=Million Btu; 1Btu=amount of energy required to raise one pound of water 1° F.) such asH-richness, lower ignition delay time, higher adiabatic flametemperature, higher flame speed, etc. It is also necessary to cope withthe much higher volumetric fuel flow rates caused by densities up to 10times smaller than for natural gas. Application of existing burnerdesigns to such fuels results in high emissions and safety problems. TheMBtu fuels, which are gaseous, cannot be injected radially into theoncoming oxidizing stream because the blockage effect of the fuel jets(i.e. stagnation zone upstream of jet, where oncoming air stagnates)increases local residence times of the fuel and promotes self ignition.Furthermore, the shear stresses are highest for a jet perpendicular tothe main flow. The resulting turbulence may be high enough to permitupstream propagation of the flame. It is important to avoidrecirculation zones around the fuel lance, which might be filled withfuel-containing gas and could lead to flashback or thermo-acousticoscillations. When injecting the fuel, it should be ensured, that thecombustible mixture is not combusted prematurely.

In a first preferred embodiment of the present invention, the fuelcontains H2 or any other equivalently highly reactive gas. A gas with asubstantial hydrogen content has an associated low ignition temperatureand high flame velocity, and therefore is highly reactive. Preferablythe fuel is synthesis gas (or Syngas), which per se is known as having ahigh hydrogen content, or any other synthetic flammable gas, as e.g.generated by the oxidation of coal, biomass or other fuels. Syngas is agas mixture containing varying amounts of carbon monoxide, carbondioxide, CH4 (main components are CO and H2 with some inert like CO2 H2Oor N2 and some methane, propane etc.) etc. and hydrogen generated by thegasification of a carbon containing fuel to a gaseous product with aheating value. Examples include steam reforming of natural gas or liquidhydrocarbons to produce hydrogen, the gasification of coal and in sometypes of waste-to-energy gasification facilities. The name comes fromtheir use as intermediates in creating synthetic natural gas (SNG). Thiskind of fuel has rather different characteristics from natural gasconcerning the calorific value, the density and the combustionproperties as e.g. volumetric flow, flame velocity and ignition delaytime. Syngas typically has less than half the energy density of naturalgas. In a gas turbine with sequential combustion, significantadjustments are thus necessary in order to cope with these differences.

According to a further embodiment of the present invention, at least aportion of the fuel is injected from the fuel lance with an axialcomponent greater than zero in flow direction with reference to thelongitudinal axis of the premixing chamber. Preferably, the radialcomponent of the fuel jet is also greater than zero. The injection holescan be inclined such that the angle of injection a of fuel from thehorizontal portion of the fuel lance between the fuel jet and thelongitudinal axis is between 10 and 85 degrees, preferably between 20and 80 degrees, more preferably between 30 and 50 degrees, mostpreferably between 40 and 60 degrees with respect to the longitudinalaxis of the premixing chamber. Preferably, the fuel jet has an axial aswell as a radial component. Fully radial injection results in excessivefuel jet/air interactions in the mixing section and thereby results in ahigh risk for premature self-ignition, whereas a fully axial injectionleads to bad mixing of fuel and air.

Another measure for improving burner safety is to re-shape thedownstream side of the fuel lance. Reducing the bluffness of thedownstream side of the lance diminishes, or even eliminates, therecirculation zone that currently exists behind this device (fueltrapped in such a recirculation zone has a very high residence time,greater than the ignition delay time).

An alternative approach achieving the same or similar or equivalenteffect, e.g. the reduction of residence time of fuel in the premixingchamber or the mixing section, respectively, would be, to inject atleast a portion (or all) of the MBtu fuel into the mixing sectionfurther downstream of the fuel lance, nearer to the burner exit, via aseries of injection holes in one or more additional injection devices(using considerations stated above, preferably with a fuel jetinclination comprising both axial and radial components) distributedalong the circumference of the mixing section tube on its periphery. Forinstance, the MBtu fuel can be supplied via a device or plenum locateddownstream of the fuel lance near the entrance to the second combustionchamber and thereby closer to the second combustion chamber than to theat least one vortex generator, which is located upstream of the fuellance. Preferably, the combustible mixture of air and fuel is createdclose to the entrance to the combustion chamber to minimise residencetime. As well as minimizing alterations of the standard fuel lance, thismethod also reduces the residence time of the MBtu fuel in the mixingsection, thereby diminishing the risk of flashback. Preferably, also theadditional injection devices have injection holes inclined in a way toenable fuel jets with axial components.

Preferably but not imperatively, the fuel lance contains more than 4injection holes. More preferably, it injects at least 8, preferably atleast 16 fuel jets into the premixing chamber The diameter of eachinjection hole is preferably reduced (while e.g. the total content offuel to be injected remains constant). This results in a greater numberof fuel jets with smaller diameters dispersed over the area of themixing section, which again results in an adapted mixing of fuel withoxidizing medium.

Furthermore, it can be of advantage, if the fuel is injected not onlywith a radial and an axial component with respect to the longitudinalaxis of the fuel lance, but also with a tangential component withrespect to the periphery of the cylindrical fuel lance tube. Dependingon whether the tangential injection of the fuel is in the direction ofswirl created in the oxidizing stream by the vortex generator(s) oragainst said swirl direction, different mixing properties can beachieved.

According to another preferred embodiment, whether or not the fuel jethas an axial component or the number of injection holes is increased orwhether or not one or more additional injection devices are providedupstream of the fuel lance, air and/or N2 and/or steam, preferably anon-oxidizing medium or inert constituent such as N2 or steam in orderto prevent back firing, can be provided as a buffer between the injectedfuel and the oxidizing stream. Such a “dilution” or shielding of thegaseous fuel improves the stability of combustion and contributes to thereduction of flashback typical for high-H2-concentrations. Preferablythe buffer is or builds a circumferential shield around the fuel jet.The carrier-/shielding properties of N2 or steam permit greater radialfuel penetration depths, which results in improved fuel distribution.The carrier provides an inert buffer between fuel jet and incomingcombustion air, such that there is initially no direct contact betweenfuel and air (oxygen) in the stagnation region on the upstream side ofthe jet. Steam is even more kinetically-neutralising than N2.Furthermore, its greater density promotes even greater fuel jetpenetration. This technique can also be employed with moreaxially-inclined jets, so as to firstly prevent contact between oxidantand fuel prior to a certain level of fuel spreading, and secondly toutilize the momentum of the carrier to increase the fuel penetration andthus improve fuel distribution throughout the burner.

For this purpose, N2 and/or steam can also be premixed with the fuelbefore injection, or can be injected separately concomitantly with thefuel or in an alternating sequence. The air and/or N2 and/or steam,preferably a non-oxidizing medium such as N2 or steam, can be injectedfrom the fuel lance itself, together or separate from the fuel, or fromone or more injection devices downstream of the fuel lance.

As already mentioned above, it can be of advantage to inject at leastsome of the fuel (with or without carrier air, N2 or steam) from thedownstream side of the fuel lance. The fuel momentum could serve toprevent the formation of any recirculation regions. If desirable, thesame effect could be achieved by injection of only air or N2 or steam.

According to another preferred embodiment, two different fuel types areinjected, preferably from different injecting devices or differentinjection locations, into the premixing chamber. A second fuel type(e.g. natural gas or oil) can serve as a backup or startup. Of course,at least one of the two fuel types is an MBtu-fuel. If the two fueltypes are injected from at least two different injection devices orlocations, at least one fuel type advantageously is injected with anaxial component with respect to the longitudinal axis of the premixingchamber.

In the sequential combustion system, it is advantageous, if the gas isat least partially expanded in a first expansion stage between the firstcombustion chamber and the second combustion chamber. In a gas turbine,said expansion preferably is achieved by a series of guide-blades andmoving-blades. Preferably, a first expansion stage is provideddownstream of the first combustion chamber and a second expansion stagedownstream of the second combustion chamber.

Alternatively, it may be of advantage if a portion of Mbtu fuel isinjected axially via the trailing edge of the vortex generators, and theremainder of the fuel via the fuel lance (using any of above concepts)and/or one or more further downstream injection devices. Apart fromimproving overall mixing and burner safety, this method frees upvaluable space in the main fuel lance, thereby permitting a second fuel(e.g. natural gas or oil) to be used as backup (or startup). In anextreme case of this alternative, all MBtu fuel is injected via thevortex generators such that the lance remains in its original guise andtherefore does not affect standard natural gas and oil operation (i.e.tri-fuel burner).

Further embodiments of the present invention are outlined in thedependent claims.

DETAILED DESCRIPTION

Referring to the drawings, which are for the purpose of illustrating thepresent preferred embodiments of the invention and not for the purposeof limiting the same, FIG. 1 shows a schematic view of a sequentialcombustion cycle with two combustion chambers or burners, respectively.The depicted arrangement can for instance make up a gas-turbine grouphaving sequential combustion, as for example having two combustionchambers of which one is coupled with a high pressure turbine and theother one with a low pressure turbine. Alternative arrangements of theunits are possible. In FIG. 1, a generator 21 is provided, which isdriven in the sequential cycle on one shaft. Air 22 is compressed in acompressor 20 before being introduced into a first combustion chamber12, followed further downstream by a first expansion stage 18. Afterpartial expansion, e.g. in a high pressure turbine, the air isintroduced into a second combustion chamber 2. Said second combustionchamber 2 can for instance be a SEV-burner, according to one preferredembodiment of the invention. Preferably, said burner takes advantage ofself-ignition downstream of the premixing chamber 4, where the air hasvery high temperatures. A second expansion stage 19 follows downstreamof said second combustion chamber 2.

FIG. 2 shows a section through of a state of the art fuel lance 5 (ase.g. in a more fuel burner). Said fuel lance 5 can be adapted to injectfuel such as oil and/or natural gas, and possibly carrier air inaddition to the fuel. The fuel lance 5 shown has at least one duct foroil 14, at least one duct for natural gas 15 and at least one duct forair 16. Said fuel lance has a vertical portion 6 and a horizontalportion 7. The horizontal portion 7 of a length L3, which is suspendedby the vertical portion 6 of a length L2 into the mixing section 17,preferably is provided with injection holes 9 for liquid fuel along acircular line around its circumference. Said injection holes 9 aregenerally provided in a downstream portion of the horizontal portion 7of the fuel lance 5, preferably in the quarter of the length L3 which islocated closest to the second combustion chamber 2. The liquid fuel isinjected radially, as described e.g. in EP 0 638 769 A2. Typically about3-4 injection holes are provided, preferably located around thecircumference in 90 or 120 degree angles from each other. In suchburners, the downstream side 8 at the tip of the fuel lance 5 is closed,i.e. it contains no injection holes 9. Therefore, the depicted fuellance 5 cannot inject fuel in an axial direction with respect to thelongitudinal axis A of the premixing chamber 4, but only radially intothe oxidizing stream 22 through the injection holes 9 depicted.SEV-burners are currently designed for operation on natural gas and oil.Besides ducts 14 for oil, the depicted state of the art fuel lance 5 isequipped with ducts 15 for natural gas and ducts 16 for air. Besidesinjection holes 9 for liquid fuel, injection holes 9 a, 9 b are alsoprovided for air and gas (e.g. natural gas) in the fuel lance 5 of FIG.2, said air and gas are injected into the combustion air radially.However, the fuel lance need not necessarily be equipped for threedifferent components. The section of FIG. 2 extends through theinjection hole 9 for oil located at the top of the horizontal portion 7of the fuel lance 5 as well as through the injection hole 9 a for airand the injection hole 9 b for gas. According to the figure, noinjection hole 9 is located 180 degrees from the top injection hole 9shown. Therefore, FIG. 2 shows a fuel lance 5 with 3 injection holes 9,such that not every injection hole 9 has a counterpart injection hole 9on the opposite side of the circumference of the fuel lance cylinder.

FIG. 3 shows a section through a part of a gas turbine group, andspecifically the part including the sequential combustion in anSEV-burner 1 according to one preferred embodiment of the invention.Said SEV-burner according to one of the embodiments of the invention isdesigned for the injection of MBtu-fuels. In such a gas turbine group,hot gases are initially generated in a high-pressure first combustionchamber 12. Downstream thereof operates a first turbine 18, preferably ahigh pressure turbine, in which the hot gases undergo partial expansion.From left to right in the figure, coming from a first burner, e.g. anEV-burner, in other words from a first combustion chamber 12 thereof,followed by a first expansion stage 18 (e.g. high pressure turbine), theoxidizing stream 22 (combustion air) enters the second combustionchamber 2 in a flow direction F. The inflow zone at the entrance to thepremixing chamber 4, which is formed as a generally rectangular ductserving as a flow passage for the oxidizing stream 22, is equipped onthe inside and in the peripheral direction of the duct wall with atleast one vortex generator 3, preferably two or several vortexgenerators 3, as depicted, or more (as e.g. described in U.S. Pat. No.5,626,017, the contents of which are incorporated into this applicationby reference with respect to the vortex generators), which createturbulences in the incoming air, followed by a mixing section 17downstream in flow direction F, into which fuel jets 11 are injectedfrom at least one fuel lance 5. The horizontal portion 7 of said fuellance 5, generally formed as a tube with a cylindrical wall 23, isdisposed in the direction of flow F of the oxidizing stream (of hot gas)22 parallel to the longitudinal axis A of the cylindrical or rectangularpremixing chamber 4 and its horizontal portion 7 preferably disposedcentrally therein. In other words, the horizontal portion 7 is disposedfrom the periphery of the duct of the premixing chamber 4 at a distanceequal to the length L2 of the vertical portion 6 of the fuel lance 5.The fuel lance 5 extends into the mixing section 17 with its verticalportion 6 suspended radially with respect to the radius of the mixingsection's cylindrical form or duct. The length L3 of the horizontalportion 7 of the fuel lance 5 is about half the length L1 of the mixingsection 17 or less.

The downstream side 8 of the horizontal portion 7 makes up the free endof the fuel lance 5 facing the second combustion chamber 2. Said freeend of the horizontal portion 7 of the fuel lance 5 can have afrusto-conical shape. This reduction of the bluffness of the downstreamside of the fuel lance 5 contributes to a reduction or elimination ofthe recirculation zone existing behind the lance. Fuel trapped in such arecirculation zone has a very high residence time, potentially greaterthan the ignition delay time.

Said two vortex generators 3 (swirl generators) are illustrated as twowedges in the figure. The hot gases entering the premixing chamber 4 areswirled by the vortex generators 3 such that mixing is possible andrecirculation areas are diminished or eliminated in the following mixingsection 17. The resulting swirl flow promotes homogenization of themixture of combustion air and fuel. The mixing section 17, beinggenerally formed as a cylindrical or rectangular duct or tube, has alength L1 of 100 mm to 350 mm, preferably 150 mm to 250 mm and adiameter of 100 mm to 200 mm. The fuel injected by the fuel lance 5 intothe hot gases that enter the premixing chamber 4 as an oxidizing stream22 initiates mixing and subsequent self-ignition. Said self-ignition istriggered at specific mixing ratios and gas temperatures depending onthe type of fuel used. For instance, when MBtu-fuels are used,self-ignition is triggered at temperatures around 800-850 degreesCelsius, whereas flashback temperature depends on H2 content. For theabove mentioned combustion chamber the main parameter which controlsflashback is ignition delay time, which goes down with increasingtemperature.

A mixing zone is established in the mixing section 17 around thehorizontal portion 7 of the fuel lance 5 and downstream of the fuellance 5 before the entrance 13 into the second combustion chamber 2, iffurther injection devices 10, as depicted in FIG. 4, are disposed on theperiphery of the mixing section 17. Preferably, the mixing zone islocated as far downstream as possible, so that the likelihood ofself-ignition on account of a long dwell time and hence the probabilityof flashback into the mixing zone is reduced.

The injection holes 9 are located on a circle line around thecircumference of the generally hollow cylindrical horizontal portion 7of the fuel lance 5. In the state of the art, the injection holes 9 arearranged in a way that the fuel is injected fully radially with respectto the axis of the cylindrical horizontal portion 7 of the fuel lance 5and/or the longitudinal axis A of the generally cylindrically shapedmixing section 17 or the premixing chamber 4. However, according to apreferred embodiment of the invention, the fuel is injected into theoxidizing stream 22 with a significant axial component in flow directionF with respect to the longitudinal axis A of the premixing chamber 4.

Said injection holes 9 can have a diameter of about 1 mm to about 10 mm.In the state of the art, the fuel lance 5 has at most 4 injection holes9. However, the fuel lance can be equipped with any number of holesbetween 2 and 32, possibly even more. In order to improve the mixingproperties, more than 4, for instance 8, or even more, e.g. up to 16 oreven up to 32 injection holes 9 can be provided on the fuel lance 5. Byincreasing the number of injection holes 9, with a constant amount offuel to be injected, the diameter of each injection hole 9 can bereduced, which leads to a more directed fuel jet 11 coming from eachinjection hole 9 and thereby to a greater injection pressure. Byachieving a more directed fuel jet 11, the fuel is distributed furtherdownstream of the fuel lance 5, thereby shifting the ignition zone to aposition further downstream and closer to the entrance 13 of the secondcombustion chamber 2. This is desired as the residence time of the fuelin the premixing chamber 4 is thereby reduced. By increasing the numberof injection holes 9 it must be noted that this measure can cause asmaller fuel penetration and consequently as a result, worse mixing.

As depicted in FIG. 4, according to another preferred embodiment of theinvention, the residence time of the fuel in the premixing chamber 4 canfurther be reduced by adding further injection devices 10 downstream ofthe fuel lance 5 in the premixing chamber 4. By injection of a portionof the fuel further downstream in the mixing section 17, the mixing zoneis shifted further downstream and closer to the second combustionchamber 2. Preferably the fuel (of one or more types) is injected fromboth the fuel lance 5 and at least one further injection device 10. InFIG. 4, only one additional circumferential injection device 10 isshown. However, more than one additional device is possible. Suchadditional injection devices 10 can be located at various positionsalong the periphery of the mixing section 17 and at different positionsdistributed along its length L1. Each additional injection device 10 canhave one or more injection holes 9, which are adapted to inject the fuelwith a radial and an axial component, at an angle α′ of about 20 to 120degrees, preferably 5-80 degrees, more preferably 30-70 degrees and mostpreferably 40-60 degrees.

Injection angle α′ is defined as the angle between the fuel jetinjection direction and the direction of the inner surface of the tubeor cylindrical wall 23, respectively, of the mixing section 17 in anaxial plane thereof. Said angle α′ can have any value of zero or greaterand at the most 180, preferably 90 degrees. The injection angle α, α′,whether from the fuel lance 5 or an additional injection devicedownstream of the fuel lance, depends on different factors, such as thetype of fuel used, whether or not a buffer such as N2 or steam isemployed, on the gas temperature etc. It is possible to provideinjection holes 9 directed at different injection angles α′ in a singleinjection device 10, such that the fuel is injected into differentdirections simultaneously. The fuel jets 11 from the additionaldevice(s) 10 can also have tangential components as discussed in FIGS. 5a.)-c.)

FIG. 5 shows a section through line B-B of the fuel lance 5 of FIG. 3.Said section extends through the injection holes 9 for fuel, i.e.through the circle line described by the injection holes around thecircumference of the fuel lance 5. Looking into the mixing section 17with its cylindrical wall 23 onto the downstream side 8 of the fuellance 5 from the second combustion chamber 2 (not shown in FIG. 5), theviewer faces the oncoming oxidizing stream 22. In FIG. 5 a.), the fueljets 11 are injected into the mixing section 17 with a radial and axialcomponent with respect to the longitudinal axis A of the premixingchamber 4, if viewed along the longitudinal axis A, but not tangentiallywith respect to the circumference of the cylindrical periphery of thefuel lance 5. The fuel jets 11 are injected along an axial plane. Inother words, the injection direction of the fuel jets 11 is not adjustedto, i.e. doesn't follow the swirl created in the oxidizing stream 22 bythe vortex generators, indicated with arrow S. If an injection directionaccording to FIG. 5 a.) is chosen, the fuel is injected along an axialplane through the injection hole 9. However, it is possible to choose aninjection direction (i.e. to adjust the injection device in the fuellance or the injection holes 9), which allows the fuel to be injected ina direction tilted out of the axial plane (see FIGS. 5 b.) and 5 c.).

In FIG. 5 a.), if viewed along the longitudinal axis A from the secondcombustion chamber 2 toward the fuel lance 5, one would see the fueljets 11 being injected radially, whereby they preferably also have anaxial component in the flow direction F with respect to the longitudinalaxis A of the premixing chamber 4. In the case of FIG. 5 a.), thetangential component is zero.

In FIGS. 5 b.) and 5 c.), the injection of the fuel jets is adjusted to,i.e. follow, the swirl of the oxidizing stream 22. The injection holes 9are arranged in a way that the fuel jets 11 are injected into the mixingsection 17 also with a tangential component greater than zero withrespect to the circumference of the cylindrical fuel lance tube. In FIG.5 b.), the tangential injection direction follows the swirl direction S,whereas in FIG. 5 c.), the tangential injection direction is opposite tothe swirl direction S. After injection, the fuel jets 11 are thendiverted to follow the swirl direction S. Depending on whether the fuelis injected tangentially in swirl direction S or against it, differentmixing properties are achieved. Intermediate injection with a tangentialcomponent is possible with angles β of 0-180 degrees, preferably 30-150degrees, even more preferably 60-180 degrees Said angle β is defined asthe angle between the injection direction and a tangential perpendicularto the radius of the cylindrical horizontal portion 7 of the fuel lance5 in a plane perpendicular to the longitudinal axis A of the premixingchamber 4.

LIST OF REFERENCE NUMERALS

-   1 SEV burner-   2 Second combustion chamber-   3 Vortex generator-   4 Premixing chamber-   5 Fuel lance-   6 Vertical portion of 5-   7 Horizontal portion of 5-   8 Downstream side of 5-   9 Injection hole for fuel-   9 a Injection hole for air-   9 b Injection hole for gas-   10 Injection device-   11 Fuel jet-   12 First combustion chamber-   13 Entrance to combustion chamber-   14 Duct in 5 for oil-   15 Duct in 5 for natural gas-   16 Duct in 5 for carrier air-   17 Mixing section-   18 First expansion stage-   19 Second expansion stage-   20 Compressor-   21 Generator-   22 Combustion air, oxidizing stream-   23 Cylindrical wall of 17-   A Longitudinal axis of 4-   F Flow direction of oxidizing air stream-   L1 Length of 17-   L2 Length of 6-   L3 Length of 7-   S Swirl direction of 22-   α injection angle in 5-   α′ injection angle in 10-   β angle of tangential component-   B-B section through 5

1. Method for fuel injection in a sequential combustion systemcomprising a first combustion chamber (12) and, downstream thereof, asecond combustion chamber (2), in between which a premixing chamber (4)having a longitudinal axis (A) comprising at least one vortex generator(3), as well as downstream of the vortex generator (3) a mixing section(17) and a fuel lance (5) having a vertical portion (6) and a horizontalportion (7) parallel to the longitudinal axis (A) provided within saidmixing section (17) is located, wherein the fuel has a calorific valueof 5-20 MJ/kg and wherein in said mixing section (17) the fuel and anoxidizing stream (22) coming from the first combustion chamber (12) arepremixed to a combustible mixture, the method comprising: injecting thefuel in such a way that the residence time of the fuel in the mixingsection (17) is reduced in comparison with a radial injection of thefuel from the horizontal portion (7) of the fuel lance (5).
 2. Methodfor fuel injection according to claim 1, wherein the fuel contains H2.3. Method for fuel injection according to claim 1, wherein the fuel hasa calorific value of 7,000-17,000 kJ/kg.
 4. Method for fuel injectionaccording to claim 1, wherein at least a portion of the fuel is injectedfrom the fuel lance (5) with an axial component in flow direction (F)with reference to the longitudinal axis (A) of the premixing chamber(4).
 5. Method for fuel injection according to claim 1, wherein an angle(α) between a fuel jet (11) injected from the horizontal portion (7) ofthe fuel lance (5) and the longitudinal axis (A) is between 10 and 85degrees, with respect to the longitudinal axis (A) of the premixingchamber (4).
 6. Method for fuel injection according to claim 1, whereina portion of the fuel is injected into the mixing section (17) from atleast one injection device (10) downstream of the fuel lance (5). 7.Method for fuel injection according to claim 6, wherein said injectiondevice (10) is located in a portion of the mixing section (17) which islocated closer to the second combustion chamber (2) than to the at leastone vortex generator (3), said portion having a length of one third orless of the length (L1) of the mixing section (17).
 8. Method for fuelinjection according to claim 1, wherein the fuel lance (5) injects atleast one fuel jet (11)
 9. Method for fuel injection according to claim8, wherein the fuel lance (5) injects at least 4, or at least 8 or atleast 16 fuel jets (11).
 10. Method for fuel injection according toclaim 1, wherein N2 and/or steam is provided as a buffer between theinjected fuel and the oxidizing stream (22), preferentially as acircumferential shielding of a fuel jet (11).
 11. Method for fuelinjection according to claim 1, wherein N2 and/or steam is premixed withthe fuel before injection.
 12. Method for fuel injection according toclaim 1, wherein air and/or N2 and/or steam is injected from aninjection device (10) downstream of the fuel lance (5).
 13. Method forfuel injection according to claim 1, wherein two different fuel typesare injected, preferably from different injecting devices (10), into thepremixing chamber (4).
 14. Method for fuel injection according to claim13, wherein two different fuel types are injected from at least twodifferent injection devices (10), wherein at least one fuel type isinjected with an axial component with respect to the longitudinal axis(A) of the premixing chamber (4).
 15. Method for fuel injectionaccording to claim 1, wherein the gas is at least partially expanded inan expansion stage (18) between the first combustion chamber (12) andthe second combustion chamber (2).
 16. Method for fuel injectionaccording to claim 1, wherein fuel is injected into the mixing section(17) of a SEV-burner (1).
 17. Method for fuel injection according toclaim 1, wherein the fuel has a calorific value of 10,000-15,000 kJ/kg.18. Method for fuel injection according to claim 1, wherein an angle (α)between a fuel jet (11) injected from the horizontal portion (7) of thefuel lance (5) and the longitudinal axis (A) is between 20 and 80degrees, with respect to the longitudinal axis (A) of the premixingchamber (4).
 19. Method for fuel injection according to claim 1, whereinan angle (α) between a fuel jet (11) injected from the horizontalportion (7) of the fuel lance (5) and the longitudinal axis (A) isbetween 30 and 70 degrees with respect to the longitudinal axis (A) ofthe premixing chamber (4).
 20. Method for fuel injection according toclaim 1, wherein an angle (α) between a fuel jet (11) injected from thehorizontal portion (7) of the fuel lance (5) and the longitudinal axis(A) is between 40 and 60 degrees with respect to the longitudinal axis(A) of the premixing chamber (4).
 21. Method for fuel injectionaccording to claim 6, wherein said injection device (10) is located in aportion of the mixing section (17) which is located closer to the secondcombustion chamber (2) than to the at least one vortex generator (3),said portion having a length of one fourth or less of the length (L1) ofthe mixing section (17).